Add #_objectFileFormat compilation conditional

[kubamracek]

This implements the relevant part of the "Section Placement" proposal (draft: https://github.com/kubamracek/swift-evolution/blob/section-placement-control/proposals/0nnn-section-control.md, forum thread: https://forums.swift.org/t/pitch-3-section-placement-control/77435). Namely the ability to conditionalize on the object file format that the compiler is targeting (MachO, ELF, COFF of Wasm).

Adding with an underscored name (#_objectFileFormat) until the proposal is codified.

rdar://147505228

[kubamracek]

@swift-ci please test

[compnerd]

Can we rewrite this as:

addPlatformConditionValue(PlatformConditionKind::ObjectFileFormat,
  llvm::StringSwitch<llvm::Triple::ObjectFormatType>(Target.getObjectFormat())
    .Case(llvm::Triple::COFF, "COFF")
    .Case(llvm::Triple::ELF, "ELF")
    .Case(llvm::Triple::MachO, "MachO")
    .Case(llvm::Triple::Wasm, "Wasm"));

[kubamracek]

Hm, not really, because we're not switching over strings here, but over the enum cases... I could do a regular switch, but that reads worse than the chain of ifs:

  StringRef FileFormatName;
  switch (Target.getObjectFormat()) {
  case llvm::Triple::COFF:
    FileFormatName = "COFF";
    break;
  case llvm::Triple::ELF:
    FileFormatName = "ELF";
    break;
  case llvm::Triple::MachO:
    FileFormatName = "MachO";
    break;
  case llvm::Triple::Wasm:
    FileFormatName = "Wasm";
    break;
  default:
    break;
  }
  if (!FileFormatName.empty()) {
    addPlatformConditionValue(PlatformConditionKind::ObjectFileFormat,
                              FileFormatName);
  }

[kateinoigakukun]

Should we consider gating this behind an experimental feature flag?

[kubamracek]

I'm happy to do that if we deem that necessary, though all the other existing not-yet-ratified compilation conditions (e.g. pointerBitWidth) don't do that and just use an underscore.

[kubamracek]

swiftlang/swift-syntax#3027

@swift-ci please test

[grynspan]

Should we consider gating this behind an experimental feature flag?

Swift Testing will need to make use of this to support Embedded Swift, and requiring an experimental flag will make it more difficult to do so. (When it gets formally featurized, we can eagerly adopt the final version.)

[grynspan]

Can we add an else clause here that diagnoses the target is unknown? That way if we add a new port, we'll know to update this code.

[kubamracek]

@swift-ci please test

[kubamracek]

swiftlang/swift-syntax#3027

@swift-ci please test